Relief Pitching? More like Unrelief Pitching

In the 5th inning the dbacks looked like they were well on there way to a 6th straight win. They were up by 7 runs and the shaking Yusmeiro Petit was doing a decent job. Predictably he began giving up home runs in the 6th, but hey it was okay we were still up 4 runs. Enter the D-backs "relief" pitching and the dbacks gave up 11 runs including 10 in one inning. Why didn't Clay Zavada pitch instead of Slaten, Vasquez, Guiterrez, Schoeneweis and Boyer? I realize that it is not healthy for Zavada to pitch everyday but we were winning and he has been the only man in the bullpen that I trust.